1. Field of the Invention
The invention is related to the field of electric heaters for the ignition of hydrocarbon fuels and in particular to glow plugs for assisting the start of Diesel type internal combustion engines.
2. Prior Art
Electrically energized glow plugs are currently used in compression ignited or Diesel type internal combustion engines to assist in the ignition of the air/fuel mixture during cold starts. In particular glow plugs are essential in the northern states during the winter months when ambient temperatures fall below 10.degree. C.
Currently the glow plugs, such as disclosed by Mann in U.S. Pat. No. 4,281,451, have a coil wire heater enclosed in a protective metal shield. The problem with these glow plugs is that they have a relatively low thermal response time, 15 to 30 seconds, and require relatively large currents, 15 to 25 amps, to bring them up to the required operating temperature. The prior art also teaches replacing the coiled wire heating element with a spiral wound flat tape type heating element as disclosed by Knowles in U.S. Pat. No. 4,297,785. Alternatively Yamamoto et al in U.S. Pat. No. 4,357,526 and Sagawa et al in U.S. Pat. No. 4,035,613 teach a discrete printed circuit heating element imbedded in a ceramic body. However, all of these glow plugs still exhibit the same slow thermal response and relatively high electrical power requirements.
In contrast to the glow plugs described above, Knoll et al in British Patent Application No. 2,092,670-A published Aug. 18, 1982, disclose a glow plug having a layered platinum-rhodium alloy surface film heater element applied to the base of a closed end ceramic tube.
The Applicant in co-pending commonly assigned Patent Application Ser. No. 430,909, filed Sept. 30, 1982 disclosed a similar glow plug having a transition metal surface film heater element circumfrentially coated on a surface of a cylindrical ceramic substrate adjacent to it's end. Tests of the glow plug disclosed in Patent Application Ser. No. 430,909 exhibited improved performance, however, contact corrosion at the opposite ends of the surface film heater element where electrical contact was made to the outer shell and axial electrical terminal were encountered during life tests. The invention is an improved embodiment of the surface film heater type glow plug which is more efficient than the glow plug disclosed by Knoll et al, is easier to make, and solves the contact problems encountered in the former design.